Cassini Spacecraft Revisits Icy Saturn Moon

NASA?s
Cassini spacecraft circling the planet Saturn took a new look at one of the
ringed world?s icy moons and its geyser-like plumes on Monday.

The probe
zoomed past Saturn?s moon Enceladus with its camera eyes wide open to
photograph the jets of
frozen water vapor that gush from fissures along the satellite?s south
pole.

?It's super
exciting, because we'll pass just about 50 kilometers (30 miles) from the surface,
and fly into the plume at the south pole,? wrote Cassini mission scientist
Amanda Hendrix of the Jet Propulsion Laboratory in Pasadena, Calif., in a NASA
blog chronicling the event.

The 39,540
mph (63,633 kph) flyby marked Cassini?s fifth swing past Enceladus, an icy moon
of Saturn about 310 miles (500 km) wide that has tantalized scientists with
geyser-like eruptions of icy water vapor that were first spotted in 2005. The
spacecraft last visited Enceladus earlier
this year in March, when it skirted through the fringe of the plumes and found
signs of organic chemicals similar to those seen in comets.

But during
today?s pass, Hendrix noted, Cassini was slated to use its cameras to take
detailed images and temperature measurements of the moon?s active jets. The
eruptions produce a halo of frozen water vapor and gas that replenishes
Saturn?s E-ring as Enceladus circles the planet.

Cassini was
expected to survey the source of the jets - prominent fractures that scientists
have dubbed ?tiger stripes? - in the infrared, visible and ultraviolet range of
the spectrum. Researchers hope the probe was able to measure the size of ice
grains within Enceladus? plumes and generate temperature maps to help determine
if liquid water or water vapor is present near the moon?s surface.

But despite
Cassini?s resolution of about 23 feet (7 meters) per pixel, the probe was not
expected to resolve
the individual vents spewing the icy water vapor, which may be up to 2 feet
(0.5 meters) wide, mission scientists said.

?We won't
see such openings even in our highest resolution images,? wrote Carolyn Porco,
Cassini?s imaging team leader at the Space Science Institute in Boulder, Colo.,
in a Sunday blog entry. ?But maybe ... just maybe ... we might see other
evidence of eruption, either something geologically unusual or banks of snow
where icy particles in the jets fall back to the ground.?

Cassini?s
Monday flyby past Enceladus began in earnest at about 3:05 a.m. EDT (0805
GMT), when the spacecraft was due to use its cameras to take the first
long-distance view of the moon?s plume of the rendezvous.

The probe was
scheduled to make its closest approach to Enceladus at about 6:31 p.m. EDT
(2131 GMT), swinging within 30 miles (50 km) of the moon, before moving on.
Cassini has orbited Saturn since July 1, 2004 and is now in the midst of a
two-year extension of its initial four-year mission.

?Whatever
we see, this will definitely be a first and very unusual event,? Porco added.

Cassini was
expected to begin beaming home the bulk of its flyby images and data at about 12:00
a.m. EDT (0400 GMT) Tuesday morning and take a parting glance at Enceladus. The
spacecraft has two more passes near Enceladus set for later this year, on Oct.
9 and Oct. 31, respectively, that could bring it even closer to the icy
Saturnian moon, NASA officials said.